1. Field of the Invention
This invention relates to a resist material having a high resistance to an oxygen containing plasma etchant and a high sensitivity to an energy beam such as an electron beam and an X-ray exposure beam, and more particularly to a resist material for use in an electron-beam or X-ray lithography process for forming patterns of submicron geometries and a method of using the resist material.
2. Description of the Related Art
In fabricating a semiconductor integrated circuit, thin film technology and photolithography technology are widely used. With a trend toward higher integration of integrated circuits, an electron beam or X-ray lithography (hereinafter, an "energy beam lithography process", such as an electron beam and an X-ray lithography process is abbreviated as e-beam/X-ray lithography) is gradually being put into practical application in fabricating LSIs or VLSIs.
In the photolithography technology, for example, in forming aluminum patterns of wirings on a semiconductor substrate, a resist layer is coated on a thin aluminum layer. The resist layer is selectively exposed to ultraviolet rays using a mask and then developed, resulting in forming resist patterns through which the aluminum layer is selectively exposed. Thereafter, the substrate is subjected to either a wet or dry etching process for removing the exposed portions of the aluminum layer. The remaining resist layer then is removed by an ashing process, and the desired aluminum pattern thus is formed on the substrate.
The above photolithography has resolution limitations of around 1 micron and, therefore, is not suitable for forming a resist pattern having dimensions less than 1 micron.
On the other hand, the e-beam/X-ray lithography utilizes irradiation of much shorter wavelength for exposure than those used in photolithography. In the e-beam lithography, an electron beam having an energy level of about 20 to 30 KeV (Kilo-electron-Volts) corresponding to about 0.1 .ANG. in wavelength is irradiated onto and swept over a resist layer. In the X-ray lithography, an X-ray source capable of radiating X-rays of several .ANG. in wavelength is utilized. Therefore, the e-beam/X-ray lithography is suitable for forming resist patterns having dimensions of less than 1 micron, in other words, to form fine patterns of submicron geometries.
There is another factor which deteriorates the quality of formed resist patterns. In some cases, a specific layer to be patterned by a e-beam/X-ray lithography process has a step structure on the substrate, the step being formed by an underlying structure, previously formed. The step has sometimes a height of 1 to 2 microns. When a single resist layer is used in lithography, fine resist patterns can not be achieved in such a case.
In order to solve the above step problem, a multilayer resist method has been developed and applied. In a simple case, a double-layer resist method is used. A first (lower) resist layer serves the function of planing the substrate surface and has a comparatively large thickness. Phenolnovolak resin or cresolnovolak resin, for example, is used as a material for the first resist layer. The material of the first resist layer is selected from those which are easily removed by a dry etching process comprising oxygen plasma. The first resist layer is coated on the substrate by a spin-coat method and has a thickness of, for example, about 2 microns, and has a flat surface. A second (upper) resist layer is coated on the first resist layer and has a thickness of about 0.2 to 0.3 microns. Material of the second resist layer is required to have a high resistance to the oxygen-containing plasma.
When the second resist layer is exposed selectively by an e-beam and developed, the remaining second resist layer patterns exhibit a high resistance during the subsequent dry etching process containing oxygen plasma. Therefore, the second resist layer patterns are transferred to the underlying first resist layer, resulting in forming first resist layer patterns similar to the second resist layer patterns.
A homopolymer of trimethylsilylmethyl methacrylate, shown in the following structural formula (1), is a known positive-type resist material which has a high resistance against reactive ion etching (abbreviated as RIE) using an oxygen-containing gas. ##STR1##
Though the above polymer exhibits a high resistance to the oxygen plasma during the dry etching, its sensitivity to the e-beam exposure is not good. To increase the sensitivity, Japanese Unexamined Patent Publication SHO-62-229141, is issued on Oct. 7, 1987 by Watanabe et al., discloses a polymer having a high sensitivity to the e-beam. A monomer disclosed in the reference, for example, has the following structural formula, wherein the alpha-methyl group of the trimethylsilylmethyl methacrylate shown by formula (1) is replaced by chlorine atom (Cl): ##STR2##
The above improved positive-type resist material has a most suitable range of e-beam irradiation intensity of less than a specified value. When the resist layer is subjected to irradiation by an e-beam of an intensity exceeding this range, the resist shows sometimes a negative-type characteristic.
The present invention provides a positive-type resist material which has a high resistance to oxygen plasma during dry etching and a high sensitivity to e-beam/X-ray irradiation and further is usable over a wide range of irradiation intensity.